1. Field of the Invention
The present invention relates to a method of preparing an iron carbide/carbon nanocomposite catalyst containing potassium for high temperature Fischer-Tropsch synthesis reaction and the iron carbide/carbon nanocomposite catalyst prepared thereby, and a method of manufacturing a liquid hydrocarbon using the iron carbide/carbon nanocomposite catalyst and a liquid hydrocarbon manufactured thereby. More particularly, the present invention relates to a technique of preparing a catalyst composed of a carbon support and both iron and potassium supported thereon wherein the carbon support is maximally uniformly impregnated with an iron hydrate via melt infiltration using a large pore volume thereof, and potassium is also uniformly supported together by means of various addition processes, including a pre-addition process for incorporating a potassium salt which is ground upon impregnation with the iron hydrate, or a mid- or post-addition process for incorporating a potassium solution using incipient wetness impregnation after impregnation with the iron hydrate, and to a technique for manufacturing a liquid hydrocarbon using the catalyst thus prepared.
2. Description of the Related Art
Fischer-Tropsch (FT) synthesis, which is a technique developed in the 1920s by German chemists Franz Fischer and Hans Tropsch, enables synthetic fuel (hydrocarbon) to be synthesized from synthesis gas (hydrogen and carbon monoxide) as represented by the following chemical scheme.(2n+1)H2+nCO→CnH(2n+2)+nH2O
In such Fischer-Tropsch synthesis, catalysts containing cobalt and iron are mainly used, and reaction conditions including reaction temperature and pressure, gas composition, etc. considerably vary depending on the kind of applied catalyst.
Fischer-Tropsch synthesis may be largely divided into, depending on the reaction temperature, low temperature Fischer-Tropsch (LIFT) in the temperature range of 200˜250° C. and high temperature Fischer-Tropsch (HTFT) in the temperature range of 300˜350° C. (Andrei Y. Khodakov et al, Chem. Rev., 2007, 107, 1672).
Typically, in the case of low temperature FT reaction, cobalt-based catalysts having a comparatively long lifetime are mainly adopted, and are advantageous because they have high activity and a long lifetime, but are problematic because they are susceptible to poisoning by sulfur compounds or the like or are very expensive compared to iron-based catalysts. Furthermore, the cobalt-based catalysts have little activity in water gas shift (WGS) and thus the ratio of synthesis gas composition (hydrogen:carbon monoxide=2:1) upon FT synthesis reaction has a great influence on the reaction.
On the other hand, iron-based catalysts are active in water gas shift, and may thus be used at various composition ratios of hydrogen to carbon monoxide in the range of 1˜2, and may also be utilized even in the presence of carbon dioxide which is an impurity gas. Hence, upon high temperature FT reaction which is carried out commercially on large scale, iron-based catalysts which are comparatively inexpensive and are resistant to poisoning by sulfur compounds have been mainly used. A typical example of high temperature FT reaction using a commercially available iron-based catalyst to date may include a Synthol process using an iron-based catalyst composed of fused iron (Fe) available from Sasol (Steynberg A. P. et al Appl. Catal. A: Gen., 1999, 186, 41).
However, fused Fe is disadvantageous because a variety of impurities may be contained upon preparation thereof and catalytic activity is low.
In the case of iron catalysts currently used in commercial processes, various promoters including potassium, copper, silica, etc., are used together to drastically improve catalytic performance. In particular, potassium is well known to increase reactivity of the catalyst in FT reaction, decrease production of methane and improve chain growth selectivity of hydrocarbon. Potassium is also well known to exhibit electronic effects as a typical base (D. C. Sorescu et al., Surf. Sci., 2011, 605, 401.; Z. P. Liu et al., J. Am. Chem. Soc., 2001, 123, 12596; S. J. Jenkins et al., J. Am. Chem. Soc., 2000, 122, 10610.). Recently, potassium is reported to partially participate in forming the active surface of iron particles (C.-F. Huo et al, Angew. Chem. Int. Ed., 2011, 50, 7403).
U.S. Pat. No. 4,340,503 discloses preparation of an iron-supported catalyst composed of a silicate support and iron and potassium supported thereon, and synthesis of C2˜C4 olefins from synthesis gas using a fluidized-bed reactor.
Preparation of iron-based catalysts using co-precipitation is performed by precipitating any iron-based precursor in the form of nitrate in the presence of an alkali which is exemplified by ammonium hydroxide. Upon using potassium oxide (K2O), it may accelerate the reaction to thus improve reaction performance. As such, potassium is appropriately used at a weight ratio of 0.1˜0.5 relative to Fe 100. Particularly, when potassium is added to the iron-based catalyst, the yield of high-boiling-point hydrocarbon having high molecular weight may be increased and the ratio of olefin and paraffin may be raised in the hydrocarbon product.
Thus, the optimal potassium concentration may favorably increase activity of FT synthesis reaction and also decrease selectivity of methane.
Below is a more detailed description of the related techniques.
Although thorough research into FT reaction has been carried out for almost 100 years since the 1920s, whether the active species of the catalyst actually participating in FT reaction is the metallic iron surface, surface or bulk iron carbide, or iron oxide is still controversial. Because activation energy is lower when carbon is diffused and thus converted into iron carbide than when FT reaction occurs on the metallic iron surface, preparation in the form of iron carbide is regarded as very important. Thereby, an induction period may be minimized, and more preferably, among various iron carbide phases, Hägg carbide (c-Fe5C2) having high activity needs to be efficiently formed (Weckhuysen, B. M. et al. Chem. Soc. Rev., 2008, 37, 2758).
As for catalysts for FT synthesis reaction, when iron-based catalysts are applied to low temperature FT reaction, catalysts resulting from a co-precipitation method have been mostly employed (Korean Patent No. 10-1087165, which discloses a method for preparing of Fe based catalyst used in Fischer-Tropsch systhesis reaction and that of liquid hydrocarbon using Fe based catalyst). This catalyst is advantageous because the amount of iron is high based on the total weight of the catalyst, thus enabling supporting of iron in a large amount.
However, this catalyst is problematic because of complicated preparation procedures, low catalyst reliability, catalyst coking due to carbon monoxide, and instability at high temperature.
Whereas, in the case of high temperature FT synthesis reaction for producing hard naphtha or gasoline, fused Fe particles have been mainly used in commercial processes, and supported catalysts using supports have been mostly applied in the research step of small laboratory scale (Qinghong Zhang et. al., ChemCatChem 2010, 2, 1030).
In the case of fused Fe used in commercial processes, it is made by being fused at a very high temperature of 1000° C. or more and is thus high in mechanical strength but undesirably has a large catalyst crystalline size and low activity (Hiroshige Matsujioto et. al., J. Catal. 1978, 53, 331).
In order to increase catalytic reduction capacity of iron catalysts, copper is used, or potassium is additionally used to increase catalytic activity.
However, methods of uniformly impregnating an iron catalyst composed of a carbon support with potassium have not yet been introduced, and there are no reports for comparing activities of such a catalyst in high temperature FT reaction depending on the amount of added potassium. In the case of the carbon support, it is stable to steam generated during high temperature FT reaction, and is favorable for heat transfer, and the inside of the carbon support may provide the atmosphere which is desirable in terms of reduction and activation of the particles, and this support may be also effective at adsorbing a carbon monoxide (CO) reactant.
Therefore, a need exists to develop techniques for preparing an iron catalyst composed of a carbon support and potassium uniformly supported thereon and for producing a liquid hydrocarbon using the same.